It is well known to scan bar code symbols on objects for purposes of automatically identifying the objects in diverse fields of use. Currently, several basic optical methods have been developed over the past three or more decades.
According to one method, bar code symbols are read by scanning a laser beam across the bar code symbol, and collecting and processing light from the return laser beam to extract information modulated onto the scanned beam by the light reflective characteristics of the bar code structure.
According to a second method, bar code symbols are read by capturing and buffering a digital image of a bar code symbol, and processing the digital image to recognize the bar code structure.
When using either method described above, the further that the object bearing the bar code symbol resides from a laser scanner, the weaker the return laser light signal will be at the time of signal detection at the photo-detector. Moreover, providing a system with the capacity to read reflective bar code symbols at near distances also increases dynamic range requirements of the system. Also, the further that the object bearing the bar code symbol resides from a digital imager, the weaker digital image intensity will be at the time of image detection. For laser scanners having a substantially large scanning, or working range, in particular, this potentially dramatic variation in signal intensity strength at the photo-detector places great demands on the electronic signal processing circuitry and its ability to deliver sufficient signal-to-noise-ratio (SNR) performance over broad dynamic ranges of input signal operation.
Consequently, great efforts have been made of the past few decades to provide laser scanning type bar code scanners, in particular, with automatic gain control (AGC) capabilities that aim to control the gain of the various analog scan data signal processing stages, regardless of the input laser return signal strength. The following U.S. Patents describe prior art efforts to date to provide automatic gain control (AGC) capabilities in laser scanning bar code symbol readers: U.S. Pat. Nos. 7,822,349; 7,172,125; 6,827,272; 6,073,848; 5,914,478; 5,701,003; 5,612,259; 5,288,983; 5,168,148; 5,148,008; 4,843,222; and 4,792,666, incorporated herein by reference as set forth herein.
In general, a feedback control is implemented in the analog domain, and the gain of an amplified stage is adjusted according a controller. The controller could be, but is not limited to, proportional control, PID control or fuzzy logic control, etc. Also, the amplifier refers to, but is not limited to preamplifier or gain stages along the signal path.
When performing middle and long range laser scanning, variable gains along the signal processing chain are desired to improve signal quality. Such multi-stage gain control is extremely important when a barcode target is located in the far field, which could be at least 30 feet away from the laser scanner.
During laser scanning bar code symbols, it is preferred that the gain is maintained substantially constant during each scan line sweep so that signal linearity is maintained, which is important for the barcode decoding. However, the AGC circuitry must have a fast response time once the object scanning distance and/or other parameters are changed. Thus, automatic gain control (AGC) suffers from a dilemma: how to maintain fast response time without sacrificing signal linearity during each scanning cycle.
Conventional analog AGC circuits have to change the gain continuously which limits the response time for automatic gain control. Moreover, the requirement of linearity during scan line generation further limits the usage of conventional AGC techniques in many applications. Also, it is known that digital AGC circuits can respond quickly between gain changes which gain adjustment significantly faster than analog-based AGC circuits.
However, despite the many improvements in AGC methods in laser scanning bar code symbol readers over the years, there is still a great need in the art for an improved laser scanning bar code symbol reading system which exhibits fast response time and signal linearity, while avoiding the shortcomings and drawbacks of prior art systems and methodologies.